Chanthaburi, the gem city

Chanthaburi sits on its river in southeastern Thailand, about 150 miles from Bangkok and close to the old ruby and sapphire ground of Pailin across the Cambodian border. Accounts of gem mining and trading here begin in the 15th century. The province was once a prolific producer of ruby and sapphire, and the trade outlasted the mines. Most Thai exporters keep their offices in Bangkok, but nearly all of the country's cutting is done in Chanthaburi, a town of about 30,000. In 2012 Thailand exported an estimated $650 million in gemstones, about half of it sapphire, according to the Gem and Jewelry Institute of Thailand.

Weekdays are quiet, with a few dozen dealer shops along narrow Si Chan Road selling loose stones, gold settings, scales, lights, loupes and tweezers. From 10 am on Friday the market spills into the neighbouring streets and alleys. Hundreds of dealers from around the country and abroad set up makeshift stands, and within half an hour the streets are full of buyers, brokers, sellers and the odd tourist. The heart of it is the corner of Si Chan and Thetsaban roads. Here the buyers sit at tables, often under handwritten signs listing what they want, and runners dash between them carrying parcels and price offers back to their bosses. Buyer and seller often share no language, so they bargain on calculators, punching in per-carat prices and counteroffers until a deal is struck. A seller watched by GIA reporters kept her rubies in the sunlight, where the colours come through richer. Trading runs until dusk, with noodles, barbecue chicken, mangoes and soup sold between the stalls.

Most of what changes hands is ruby and sapphire in every colour, some for as little as $2 or $3 a carat, alongside spinel, tourmaline, tsavorite and spessartite garnet, amethyst and citrine. Asked whether treatments are disclosed, one dealer said the price is the disclosure. On the side streets behind the market dozens of burners heat and otherwise treat stones, most of them with no sign out front. Diffusion treatment, which turns unsellable corundum into low-cost yellow sapphire, gave the trade a lift and put at least 10 mine sites back to work near Khao Ploy Waen and Ban Ka Cha. One site produces black star sapphire that is not treated.

The town has other landmarks. The Cathedral of the Immaculate Conception is the largest church in Thailand, and the monument to King Taksin the Great in Tungnachaey park appeared on the back of the 20 baht note issued in December 1981.